How to echo a MySQLi prepared statement?

I don't think you can - at least not in the way that you were hoping for. You would either have to build the query string yourself and execute it (ie without using a statement), or seek out or create a wrapper that supports that functionality. The one I use is Zend_Db, and this is how I would do it:

$id = 5;
$baz = 'shazam';
$select = $db->select()->from('bar','foo')
                       ->where('id = ?', $id)
                       ->where('baz = ?', $baz); // Zend_Db_Select will properly quote stuff for you
print_r($select->__toString()); // prints SELECT `bar`.`foo` FROM `bar` WHERE (id = 5) AND (baz = 'shazam')

I have struggled with this one in the past. So to get round it I wrote a little function to build the SQL for me based on the SQL, flags and variables.

//////////// Test Data //////////////
$_GET['filmID'] = 232;
$_GET['filmName'] = "Titanic";
$_GET['filmPrice'] = 10.99;

//////////// Helper Function //////////////
function debug_bind_param(){
    $numargs = func_num_args();
    $numVars = $numargs - 2;
    $arg2 = func_get_arg(1);
    $flagsAr = str_split($arg2);
    $showAr = array();
    for($i=0;$i<$numargs;$i++){
        switch($flagsAr[$i]){
        case 's' :  $showAr[] = "'".func_get_arg($i+2)."'";
        break;
        case 'i' :  $showAr[] = func_get_arg($i+2);
        break;  
        case 'd' :  $showAr[] = func_get_arg($i+2);
        break;  
        case 'b' :  $showAr[] = "'".func_get_arg($i+2)."'";
        break;  
        }
    }
    $query = func_get_arg(0);
    $querysAr = str_split($query);
    $lengthQuery = count($querysAr);
    $j = 0;
    $display = "";
    for($i=0;$i<$lengthQuery;$i++){
        if($querysAr[$i] === '?'){
            $display .= $showAr[$j];
            $j++;   
        }else{
            $display .= $querysAr[$i];
        }
    }
    if($j != $numVars){
        $display = "Mismatch on Variables to Placeholders (?)"; 
    }
    return $display;
}

//////////// Test and echo return //////////////

echo debug_bind_param("SELECT filmName FROM movies WHERE filmID = ? AND filmName = ? AND price = ?", "isd", $_GET['filmID'], $_GET['filmName'], $_GET['filmPrice']);

I have also build a little online tool to help.

Mysqli Prepare Statement Checker